The invention relates generally to gas turbine engines, and, more specifically, to microchannel cooling therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in a high pressure turbine (HPT), which powers the compressor, and in a low pressure turbine (LPT), which powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
Engine efficiency increases with temperature of combustion gases. However, the combustion gases heat the various components along their flowpath, which in turn requires cooling thereof to achieve a long engine lifetime. Typically, the hot gas path components are cooled by bleeding air from the compressor. This cooling process reduces engine efficiency, as the bled air is not used in the combustion process.
Gas turbine engine cooling art is mature and includes numerous patents for various aspects of cooling circuits and features in the various hot gas path components. For example, the combustor includes radially outer and inner liners, which require cooling during operation. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all of these exemplary gas turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. For example, a series of internal cooling passages, or serpentines, may be formed in a hot gas path component. A cooling fluid may be provided to the serpentines from a plenum, and the cooling fluid may flow through the passages, cooling the hot gas path component substrate and coatings. However, this cooling strategy typically results in comparatively low heat transfer rates and non-uniform component temperature profiles.
Microchannel cooling has the potential to significantly reduce cooling requirements by placing the cooling as close as possible to the heat zone, thus reducing the temperature difference between the hot side and cold side for a given heat transfer rate. However, current techniques for forming microchannels typically require the use of a sacrificial filler to keep the coating from being deposited within the microchannels, to support the coating during deposition, as well as the removal of the sacrificial filler after deposition of the coating system. However, both the filling of the channels with a fugitive material, and the later removal of that material present potential problems for current microchannel processing techniques. For example, the filler must be compatible with the substrate and coatings, yet have minimal shrinkage, but also have sufficient strength. Removal of the sacrificial filler involves potentially damaging processes of leaching, etching, or vaporization, and typically requires long times. Residual filler material is also a concern.
It would therefore be desirable to provide a method for depositing coatings on hot gas path components with grooves to form cooling channels therein that eliminates the need for the filling and removal processes. In addition, it would be desirable to provide a method for depositing coatings on hot gas path components that reduces the amount of coating deposited on the interior or exposed wall surfaces of the channels when deposited without the use of a sacrificial filler.